There is often a requirement in fiber optic system design for precise control of the optical signal levels entering various system components. This is particularly true when a deployed fiber optic system requires tailoring for optimum performance as a final stage. An adjustable attenuator, which is set at a desired level of attenuation and remains stable with time, temperature, etc. is an important part of this tailoring stage.
The majority of fiber optic adjustable attenuator devices currently commercially available rely on controlled air gaps between polished fibers. The attenuation level is adjusted by mechanically separating the fiber ends, and reducing the fraction of light captured by the pick-up fiber. Certain steps must be taken to provide acceptable levels of back-reflected light and avoid in-line etalon affects due to reflections from the polished fiber ends. Often this requires anti-reflection coatings or angle-polishing of the fiber ends that add to the cost and fabrication complexity of the device.
Therefore, there is a need for a fiber optic adjustable attenuator device architecture that keeps the optical fiber core intact and adjusts attenuation by selectively inserting a glass preform of desired dimensions and refractive index into an evanescent field of, e.g., a side-polished fiber.